Treatment of fibrous insulating materials



Patented Sept. 7, 1937 I UNITED STATES PATENT OFFICE TREATMENT OF FIBROUS INSULATING MATERIALS Robert R. Williams, Roselle, N. J., assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York No Drawing; Original application November 13,

1925, Serial No. 68,914. Divided and this application November 26, 1927, Serial No. 236,023. In Belgium May 14, 1926 18 Claims. (Cl. 204-1) This invention relates to the art of preparing or properties under rising humidity conditions fibrous materials for use as insulating materials therefore greatly improved since pure cellulose and is a division of my application Serial No. contains substantially no electrolytic materials 68,914, filed November 13, 1925, which has become which are dissociated into ions in the presence of 5 abandoned. moisture. Substantially the same principle ap- 5 An object of the invention is to improve the plies to animal fibres which are non-cellulose electrical insulating characteristic of fibrous mamaterial. terials. This invention provides a cheap and efllcient' Another object of the invention is to improve fibrous material whose insulating characteristics the quality and reduce the cost of fibrous insuhave been improved without sacrificing appreci- 10 lated conductors. ably either strength or flexibility.

It is well known that different insulating ma- In accordance with a feature of the invention teliels y e y in their electrical insulati g there is provided a new and improved method of characteristics such as direct current insulation improving the electrical insulating characterisresi effective electrostatic c p ity, and altics of fibrous material by reducing the ionizable 15 ternating current conductance or leakance. Fimatter content of fibrous materials by means of brous materials such as ordinary co ton d t an electric current. In accordance with another sah floss ilk p ss o ins la in eh feature of the invention the electrical insulating istics wh y but it has been found t Where characteristics of fibrous material are improved these materials are exposed to moist air, their by suitable washing 01- ieaching with water to 20 insulation resistance is considerably lowered rereduce th ionizable tt t t of th ga d ess of their relative insulating p p terial. In accordance with still another feature The insulat ac st s o all c of the invention, the insulating characteristics terials fall Off p y under rising humidi y of fibrous materials are improved by treating the 5 dltions. I material to remove the ionizable substance con- 5 While the amount of water absorptio is not tained therein and treating with a solution conneecssarily a distinct and proportionate measure taining bstance which will be appreciably of the insulation resistance of fibrous materials, ad rbed by th surface f the individual fibres, neverth less the presence f water pr s adel- In accordance with the feature of the invention etel'ious efi t po the insulating fl first mentioned, the insulating characteristics of 30 tics. Water acts as a solvent in which the vafibrous materials, such as cotton and tussah floss rious electrolytic substances contained in the silk, may be improved by electrolytically reducing fibrous material'are dissociated into ions which the ionizable matter content of the material to are electrically conducting, and thereby decreases roduce a substantially pure fibrous material and the insulating characteristics of the material. one possessing high insulating characteristics 35 Heretofore, the use of cotton as an electrical over h al range of humidity conditi insulating material has been limited to those It has been found that when a direct current cases where only a moderate degree of insulation voltage is maintained in high humidities on a was desired. In cases where a. higher insulation condu t r having fibrous insulating o ri 0 was desired, the cotton was either replaced with there result progressive changes in the insulat- 40 some medium having hi er insulating a ing characteristics of the fibrous covering tendterlstics or combined with some additional insuing toward improvement. This improvement has lilting medium such as rubber or a varnish combeen found to be associated with a gradual depleposition. While in such cases higher insulating tion of the electrolyte constituents in the middle characteristics were produced, flexibility of the portion of the conducting path and an accumu- 45 fibrous material was sacrificed or other difl'lcullation of the ash content in the vicinity of the ties were introduced such as excessive size of electrodes. Asuitable method of treating fibrous conductor or a tendency to corrosion. materials in the form of bolts or cops, for ex- It is commonly known that fibrous insulating ample, is to apply an electric potential between material such as cotton, contains foreign matter an electrode in contact with the center of the 60 which ispartly represented by the ash remainbolt and a second electrode in contact with the ing after thorough ignition of the material. It outer surface of the bolt for a suitable length of has been found that by the removal of such time in water or in ahumid atmosphere. foreign matter cotton can be made to approach The voltage applied in any particular case will pure cellulose, and its insulating characteristics depend primarily upon the size of the bolt and 55 the ash content of the material. With a given applied voltage, the duration of treatment will depend primarily upon the current leakage or current flow between the electrodes. With such 5 a treatment, the ash content of the material being treated migrates to the vicinity of the electrodes whence it may be removed by continuous washing, leaving the intervening part of the material substantially free of ash. In accordance with a modification of this feature of the invention the ionizable matter content of the fibrous material is reduced by washing or leaching with water at a suitable temperature and for a suitable length of time. The water employed for the washing operation is preferably free from the ordinary electrolytic materials in order to prevent the occurrence of undesirable electrolytic materials in the finished product which would,

tend to decrease the insulating characteristics of the material. Pure water has been found satisfactory for this purpose.

A satisfactory method comprises washing the fibrous material with pure water for a period of time varying from one to three hours at a temperature varying from 80 C. to 100 C. However, the length of time and temperature of treatment will be dependent upon the rate of flow of water over the material, the amount of water employed and the ash content of the material. The

0 values of these factors are preferably chosen so as to provide a finished product which is substantially free of foreign matter or ash. The term ash is used interchangeably with foreign matter since it has been found that when the product is substantially free from ash, objectionable electrolytes are also absent.

After the ash content of the fibrous material has been reduced in either of the ways described above, the insulating characteristics may be further improved by treating the purified material with a solution containing a substance which is relatively irreversibly adsorbed on the surface of the fibres such as a solution containing compounds of the alkaline-earth metals. By the term 46 irreversibly adsorbed is meant adsorption in which the material adsorbed is substantially fast, the term "fast having the same meaning as when applied to dyes. Sodium chloride is an example of a material which is not fast, for

50 although it is adsorbed by fibrous materials it is readily and quickly removable by washing with water. Calcium hydroxide, however, when adsorbed on the surfaces of fibres, is not readily removed by washing and is fast as an adsorbed material. By such a treatment, experiments have shown that the insulating characteristics of cotton, for example, may be made comparable to those of commercial silk, under high humidity conditions.

Lime water has been found to be an advantaeous substance for use as the solute in asolution for the treatment of a. purified material. Solutions substantially saturated with lime water have been found to be satisfactory. Other solutes 5 which have been found satisfactory are the carbonates and hydroxides of the alkaline-earth metals. The term alkaline-earth metals is employed herein to designate the metals barium, strontium and calcium.

The choice of the metal and the anion to be employed in any given case will be determined primarily by the particular conditions prevailing in that case. As a general rule it has been found that adsorption increases with the size of the. cation and with increasing valence. The

- riod of two or three minutes.

anion should preferably be one forming a volatile or non-acidic substance in water solution so that washing subsequent to the adsorption treatment will not be necessary. i

The time and temperature of treatments have been found to vary with the solute employed, those solutes which are more strongly adsorbed necessitating shorter periods of treatment. In any case, it has been found, however, that the maximum adsorption is approached rapidly, us-

ually only a few minutes treatment being required. A method which has been found satisfactory is to immerse the material to be treated in a saturated solution of calcium' hydroxide (lime water) at a temperature of 20 C. for a pe- Longer immersing periods ranging up to two or three hours, for

,example, have not however been found to result in any injurious effect to the fibrous material.

In its preferred form, the invention comprises a method of treating fibrous materials to improve their insulating characteristics which is a combination of an initial purification and a subsequent adsorption treatment into one step and in which the fibrous material is immersed in a solution containing compounds of the alkalineearth metals and between electrodes immersed in the solution and an electric current is passed between the electrodes for a suitable length of time. After a suitable length of treatment, the fibrous material is removed and dried. As described above, this treatment produces fibrous material having optimum electrical characteristics at certain definite points with respect to the electrodes. It has been found that the location of these definite optimum positions varies with the solute used in the electrolytic bath and must be determined for each material. When lime water is used in the electrolytic bath, for example, the optimum electrical characteristics have been found to be associated with the fibrous material lying mid-way between the electrodes.

The particular working conditions, such as duration and temperature of treatment, and the value of the current employed will vary with the solute used and the amount and quality of material to be treated. One method which has been found satisfactory is to apply a potential of volts for twenty-four hours at a temperature of 25 C. when using a bath of saturated lime water 10 inches in length between electrodes. The resistance rises rapidly during the early portion of the treatment so that the current consumption in the later stages is very small. Another satisfactory method of treating fibrous materials to improve their insulating characteristics comprises washing the fibrous material for a period of time of sufficient duration to cause an appreciable reduction in the ash content with a water solution containing substances which are adsorbed by the surfaces of the fibres being treated, such as water solutions of compounds of the alkali earth metals. A solution whose adsorbing ion concentration is low may be satisfactorily employed in this process since the duration of treatment is made comparatively long to allow for a thorough washing of the fibres. Natural waters which contain an appreciable amount of compounds of the alkali earth metals and are relatively free from other salts have been found to be suitable for use in such a treatment.

The duration and temperature of the treatment will be dependent substantially upon the operating conditions of the plant. With one of these factors constant, however, the other facemployed, it being preferable to provide for an efilcient circulation of water in and through the fibres.

A suitable method is to pass hot natural water of the above description through spools of insulating cotton in a machine of the type of the Franklin dyeing machine for a period ranging from one to three hours. After this time the material may be removed from the machine and dried when it is in condition for use as an improved insulating medium.

Tests have been made which illustrate the efficiency of insulating materials prepared in any of the manners herein described. It has been found, for example, that the direct current insulation resistance of cotton may be increased from 20 to times, the effective electrostatic capacity decreased between 25 and 50% and the conductance or leakance measured with an alternating current of 1000 cycles is decreased to 25 about 1/100 of the value before treatment. It

will be seen therefore that the electrical characteristics of insulating materials prepared in accordance with the invention will be materially improved and that telephone or switchboard cords insulated with materials treated in this manner are particularly satisfactory under the conditions of atmosphere which are encountered in telephone exchanges.

A particular advantage of the invention is the conversion of cotton, which is cheap but which possesses poor insulation resistance in moist air, into a condition such that it has insulating characteristics in moist air comparable to those of commercial silk under severe humidity conditions. While the invention has been described with particular reference to cotton, it is to be understood that it is applicable to other types of fibrous materials having the electrical or adsorption characteristics of cotton. By adsorption characteristics of a fibrous material is meant the property of the material which permits it to irreversibly adsorb ions, such as alkaline earthmetal ions, in the manner described herein above.

The invention is also capable of other modifications and adaptations not specifically referred to but included within the scope of the appended claims. What is claimed is: 1. The method of treating cotton to improve its insulating characteristics which comprises washing said cotton in an aqueous solution containing a calcium compound and substantially free from all other ionizable substances.

2. The method of treating cotton to improve its insulating characteristics which comprise immersing said cotton in a bath of saturated lime water maintained at a temperature of approximately 25 C. for "a period of 24 hours and impressing a potential of approximately 110 volts between electrodes spaced approximately 10 inches apart in said bath and between which the cotton is positioned.

3. A fibrous insulating material comprising long cotton fibres substantially free from electrolytic substances, the surfaces of said fibres containing cations of an alkaline earth metal which have been irreversibly adsorbed thereby from a water solution containing an alkaline earth metal and substantially free of other electrolytes.

4. The method of treating cotton material to improve its insulating characteristics which comprises washing the material with a water solution containing a compound of an alkaline earth metal and substantially free of other electrolytes.

5. As a new article of manufacture, electrical insulatirm material comprising strands of cotton substantially free from ionizable impurities.

6. As a new article of manufacture, electrical insulating material comprising cotton strands which are substantially free from ionizable substances and which have electrical insulating characteristics comparable to those of commercial silk.

7. As a new article of manufacture, electrical insulating material comprising cotton which is substantially free from ionizable substances and the surfaces of the individual fibres of which contain a compound of an alkaline earth metal which is irreversibly adsorbed thereby.

8. The method of treating fibrous material having the adsorption characteristics of cotton to improve its insulating characteristics without materially altering its fibrous structure, which comprises removing electrolytically the ionizable substance contained therein and subsequently immersing said material in a solution containing a compound of an alkaline earth metal and substantially free of other electrolytes.

9. The method of treating fibrous material having the adsorption characteristics of cotton to improve its insulating characteristics, which comprises immersing said material in an electrolyte containing alkaline earth cations which are irreversibly adsorbed by said fibrous material and substantially free from other cations and passing electric current through said electrolyte 11. The method of treating fibrous material having the adsorption characteristics of cotton to improve its insulating properties which comprises removing the ionizable impurities therefrom by washing said material for a period from one to three hours with hot water containing an alkaline earth compound and relatively free from readily ionizable substances.

12. The method of improving the insulating properties of fibrous material having the adsorption characteristics of cotton, which comprises washing the material with lime water which is substantially free of other electrolytes.

13. The method of improving the insulating properties of cotton, which comprises washing the cotton in hard water containing an appreciable amount of compounds of alkaline earth metals and which is relatively free from readily ionizable impurities until substantially all ionizable impurities are removed from the cotton.

14. The method of improving the insulating properties of cotton, which comprises washing the cotton with lime water which is substantially free of other electrolytes.

15. Electrical insulating material comprising fibrousmaterial having the adsorption characteristics of cotton characterized by being substantially free from ionizable substances and by having a compound of an alkaline earth metal irreversible adsorbed on the surfaces of the individual fibres of the material.

16. Electrical insulating material comprising cotton strands characterized by being substantially free fromvionizable impurities and by having electrical insulating characteristics under conditions of high humidity of the same order as those it possesses under ordinary conditions.

17. The method of improving theiinsulating properties of commercial insulating cotton, which comprises washing said material with water which is substantially free from readily ionizable substances until substantiallyall ionizable impurities are removed therefrom.

18. An insulating material comprising spun cotton which has substantially constant electrical characteristics under varying conditions of humidity which has a calcium compound irreversibly adsorbed thereby, and which has been freed 10 from substantially all impurities.

ROBERT R. WILLIAMS. 

